The effect of psychological treatment on social functioning, quality of life and autism traits in autistic people with suicidal behaviour remains largely unknown. At six Dutch mental health centres, 123 adult outpatients with "Diagnostic and Statistical Manual of Mental Disorders" (5th edition) diagnosed autism spectrum condition and suicidal behaviours were randomly assigned to dialectical behaviour therapy (n = 63) or treatment as usual (n = 60) to address their suicidal behaviours. This article analysed secondary outcomes on social functioning, quality of life and autism traits. Outcomes were compared at baseline, post-treatment at 6 months, and 12-month follow-up. At post-treatment, both social functioning (p < 0.001) and quality of life (p = 0.002) were significantly improved in the treatment condition compared to the control condition and remained so at 12-month follow-up (p = 0.003; p = 0.002). Autism traits did not differ between conditions. Autistic individuals with suicidal behaviours benefit modestly from treatment with dialectical behaviour therapy in social functioning and quality of life.